1. Field of the Invention
The present invention relates to a reproduced signal evaluation method for an optical disc medium in which recording marks having physical properties different from other portions are formed on a recording medium to store information, and a write adjustment method of adjusting conditions for writing information.
2. Description of the Related Art
A technology for binarizing a reproduced signal with an adaptive equalization technique and a PRML (Partial Response Maximum Likelihood) technique has been becoming essential along with the emergence of optical discs with higher speed and higher density. The densification of an optical disc makes recording marks small in size relative to an optical spot, and accordingly reduces the amplitude of a reproduced signal obtained therefrom. The resolution of an optical spot is determined by a wavelength λ and the numerical aperture NA of an objective lens. When the length of a recording mark of a minimum run length is λ/4 NA or less, the amplitude of repetitive signals reproduced therefrom is zero. This is a phenomenon generally known as an optical cut-off effect. In the case of a Blu-ray Disc (hereinafter, BD), λ/4 NA≈119 nm. If the BD is designed to have a storage capacity of approximately 31 GB or more with a track pitch set constant, repetitive signals of 2T, which is the minimum run length, have an amplitude of zero. Thus, the adaptive equalization technique and the PRML technique need to be used to obtain good reproduction performance of an optical disc under such high density conditions.
As for a write-type optical disc, a laser beam intensity-modulated in a pulse shape (hereinafter, referred to as write pulses) is used to change a crystalline state or the like of a recording film, thereby recording desired information on the disc. The recording film is a widely known film made of a material such as a phase change material, an organic dye, or a certain type of alloy or oxide. In a mark edge code scheme used in the BD, code information is determined by the positions of front and ending edges. Among the write pulses, a first pulse, a last pulse and a cooling pulse are important in terms of their positions and widths to maintain the quality of recorded information in good condition. Here, a condition of forming the leading edge of the recording mark is determined mainly based on the first pulse, and a condition of forming the ending edge of the recording mark is determined mainly based on the last pulse. Therefore, write pulses generally used for the write-type optical disc are adaptive write pulses in which the first pulse, the last pulse and the cooling pulse are adaptively changed in position or width depending on the length of a recording mark and the length of its preceding or following space.
Since finer recording marks are formed under the high density conditions described above, irradiation conditions for write pulses (hereinafter, referred to as write conditions) need to be determined with higher accuracy than ever before. Meanwhile, the shape of the optical spot produced by an optical disc device varies depending on factors such as a wavelength, wave aberration and focus conditions of a light source, and tilt of a disc. In addition, since the impedance and quantum efficiency of a semiconductor laser device vary according to an ambient temperature and with aged deterioration, the shapes of write pulses also vary. Trial writing is a general term indicating an adjustment technique for always obtaining the optimum write conditions according to the shape of the optical spot and the shapes of the write pulses, both of which vary depending on individual devices or discs and environments, as described above. With an increase in recording density, greater importance is placed on the write adjustment technique.
As a method of statistically evaluating the quality of recorded data, which is applicable to the PRML technique, there are methods described in Non-Patent Document 1 “Jpn. J. Appl. Phys., Vol. 43, pp. 4850 (2004),” Patent Document 1 “JP 2003-141823 A,” Patent Document 2 “JP 2005-346897 A,” and Patent Document 3 “JP 2003-151219 A.” In addition, the technique described in Document 4 “JP 2005-196964 A” provides an evaluation index of the quality of a reproduced signal, which is applicable to the PR(1,2,2,2,1)ML technique.
In Non-Patent Document 1 “Jpn. J. Appl. Phys., Vol. 43, pp. 4850 (2004),” a maximum likelihood sequence error (MLSE) is defined by using a reproduced signal and two target signals. Specifically, a first one of the target signals is a first binary bit array (equivalent to the most likely state transition sequence) obtained from the reproduced signal, whereas a second target signal is a second binary bit array (equivalent to the second most likely state transition sequence) in which a focused edge in the first binary bit array is shifted by one bit. A first Euclidean distance (equivalent to Pa) between a reproduced signal and the first target signal and a second Euclidean distance (equivalent to Pb) between the reproduced signal and the second target signal are obtained. The MLSE is defined as a value obtained by subtracting a Euclidean distance between the two target signals from the absolute value of a difference between the first Euclidean distance and the second Euclidean distance. Then, the write conditions are adjusted so that the mean value of the distribution of MLSE can be zero for each write pattern.
Patent Document 1 “JP 2003-141823 A” discloses a technique of evaluating the quality of a reproduced signal on the basis of the distribution of |Pa−Pb| where Pa denotes a likelihood of the most likely state transition sequence, and Pb denotes a likelihood of the second most likely state transition sequence.
Patent Document 2 “JP 2005-346897 A” focuses attention on an edge shift, and discloses a technique of adjusting write conditions. In this technique, a pattern including a virtual 1T run length is generated as an error pattern in which an edge in the reproduced signal is shifted to the right or left, and a sequence error difference with a sign depending on an edge shift direction is calculated by using the error pattern. Then, an edge shift amount is calculated based on the calculated signed-difference, and the write conditions are adjusted so that the edge shift amount can be as close to zero as possible. This evaluation index is called V-SEAT (Virtual state based Sequence Error for Adaptive Target).
In the technique disclosed by Patent Document 3 “JP 2003-151219 A,” an error probability that a focused edge is shifted to the left and an error probability that the focused edge is shifted to the right are calculated on the basis of Euclidean distances between a reproduced signal and both of a correct pattern and an error pattern, and then write conditions are adjusted so that the two error probabilities can be equal to each other. Accordingly, this technique uses a predetermined reproduced signal, a first pattern corresponding to a signal waveform pattern of the reproduced signal, and another certain pattern (a second or third pattern) being different from the first pattern but also corresponding to the signal waveform pattern of the reproduced signal. Firstly, calculation is made to obtain a distance difference D=Ee−Eo between a distance Eo between the reproduced signal and the first pattern, and a distance Ee between the reproduced signal and the certain pattern. Then, the distribution of the distance differences D is obtained from multiple samples of reproduced signals. Thereafter, a quality evaluation parameter (M/σ) of the reproduced signal is determined on the basis of a ratio of the mean value M of the obtained distance differences D to the standard deviation a of the obtained distribution of the distance differences D. Then, the quality of the reproduced signal is judged based on an evaluation index value (Mgn) represented by the quality evaluation parameter.
Patent Document 4 “JP 2005-196964 A” discloses a technique using a table in which pairs of correct patterns and corresponding error patterns are stored in advance, calculating a difference between a Euclidean distance between a reproduced signal and each correct pattern and a Euclidean distance between the reproduced signal and the corresponding error pattern, and obtaining a simulated bit error rate (SbER) from the mean value and the standard deviation of the calculated differences.    Patent Document 1: JP 2003-141823 A    Patent Document 2: JP 2005-346897 A    Patent Document 3: JP 2003-151219 A    Patent Document 4: JP 2005-196964 A    Non-Patent Document 1: Jpn. J. Appl. Phys., Vol. 43, pp. 4850 (2004).